Multiple-input and multiple-output (MIMO) technologies are widely applied in a Long Term Evolution (LTE) system. A transmitting end may use a precoding technology to process data, so as to improve signal transmission quality or a signal transmission rate. The transmitting end may be a base station or a terminal device.
In the LTE system, a base station obtains a precoding matrix usually in a manner in which a terminal device feeds back a precoding matrix indicator (PMI). A set of used precoding matrices is usually referred to as a codebook or a codebook set. Each precoding matrix in the codebook or the codebook set may also be referred to as a codeword.
The terminal device quantizes channel state information (CSI) and feeds back the CSI to the base station. The base station determines a precoding matrix based on the CSI. CSI information fed back in an existing LTE system includes a rank indication (RI), a PMI, and the like. The RI is used to indicate a quantity of data streams for spatial multiplexing. The PMI is used to indicate a precoding matrix.
An LTE standard supports 8 antenna ports and 16 antenna ports. Currently, a dual-stage codebook feedback mechanism is defined to reduce a quantity of bits of a PMI fed back by a terminal device, so as to reduce load. First-stage feedback indicates a vector group, including four vectors for subsequent processing. One of the four vectors is selected for second-stage feedback, and the selected vector may be used for data precoding. The first-stage feedback has a long period/a wide band characteristics, and the second-stage feedback has a short period/a sub-band characteristics.
FIG. 1 is a schematic diagram of a two-dimensional antenna array. In FIG. 1, an antenna port has 45 degrees polarization and −45 degrees polarization. For a polarization direction, there are N1 antenna ports in a horizontal direction, and there are N2 antenna ports in a vertical direction.
As a quantity of antenna ports increases, a beam width obtained after the base station performs precoding becomes increasingly narrow. To better cover an entire system bandwidth, the vector group in the first-stage feedback needs to include more vectors. For example, for 32 antenna ports, 16 antenna ports are included in each polarization direction. To enable a vector group in first-stage feedback in a system with 32 antenna ports and a vector group in first-stage feedback in a system with 16 antenna ports to have same coverage space, the vector group in the first-stage feedback in the system with the 32 antenna ports needs to include 16 column vectors, as shown in FIG. 2a and FIG. 2b. 
In a case of the 32 antenna ports, if the current codebook feedback mechanism is still used, to be specific, the vector group in the first-stage feedback includes four vectors, a coverage bandwidth is inevitably affected, leading to performance degradation of a codebook.
If a quantity of vectors in the first-stage feedback is increased to 16, the 16 vectors need to be used for selection for the second-stage feedback. This increases a quantity of bits in the second-stage feedback, and consequently, increases system overheads of the terminal device.